This invention relates generally to pneumatic tires and in particular to pneumatic tires for heavy duty use comprising a radial carcass and two bead regions each reinforced by a bead core whereby the carcass comprises two sets of at least two textile plies each, anchored around the bead core, the plies of the first set being wrapped around the bead core from the inside to the outside of the tire and the plies of the second set being wrapped around the bead core from the outside to the inside of the tire and whereby the bead regions each comprise an axially inner and an axially outer textile reinforcing layer and a rubber apex strip of a substantially triangular cross-section extending radially outwardly from the bead core, the axially inner and outer textile reinforcing layer being respectfully disposed axially inside and outside of said first and second sets of carcass plies along the radial extent of the rubber apex strip, the first and second sets of carcass plies being separated from one another only below a separation point at the apex of the rubber apex strip.
Radial carcass tires for heavy duty use are frequently used under heavy load conditions which place the components within a tire under severe stress. The bead regions of such tires are particularly subject to large deformations caused by flexing of the sidewalls which can produce sheer stresses between the carcass plies, buildup of heat in the tire and subsequent separation failure.
The applicants have discovered a tire construction with particularly durable bead regions which reduces the sheer stresses between the carcass plies. The pneumatic tire of the present invention is characterized in that the height of the radially outermost end of the rubber apex strip is located between 0.15 and 0.2 times the height of the tire cross-section, and in that the ply endings of the first set of carcass plies are located at a height which is lower than the height of the radially outermost end of the rubber apex strip, and in that the height of the radially outermost edge of the axially inner textile reinforcing layer is greater than the height of the radially outermost edge of the axially outer textile reinforcing layer, the height of the radially outermost edge of the axially inner textile reinforcing layer being located between 0.25 and 0.45 times the height of the tire cross-section and the height of the radially outermost edge of the axially outer textile reinforcing layer being located between 0.2 and 0.35 times the height of the tire cross-section.
The construction according to the present invention provides a separation point between the first and second set of carcass plies which is situated outside the flexing zone of the tire. The separation point between the first and second set of carcass plies is determined by the height of the radially outermost end of the rubber apex strip. Since the first and second set of carcass plies follow the same path in the flexing zone and separate from one another only in the area protected by the axially inner and axially outer textile reinforcing layers, the sheer stresses between the first and second set of carcass plies are reduced.
The axially inner and axially outer textile reinforcing layers are disposed such that the required stiffness gradient in the bead region is retained, the stiffness decreasing gradually radially outwardly from the radially outermost end of the rubber apex strip into the flexing zone of the tire.
In a preferred embodiment of the invention, the axially inner and axially outer textile reinforcing layers comprise square woven fabric which help towards reducing the ply waviness of both the reinforcing layers and the carcass plies.
The textile reinforcing layers are generally composed of one or more textile plies having parallel cords making substantially the same angle with the mid-circumferential plane of the tire. During the first stage of the tire building process, these plies are typically applied onto the carcass band on a flat building drum, and subsequently during the tire shaping stage, the building drum is made to expand to conform the tire casing to its final toroidal shape. During this expansion, the textile reinforcing plies in the bead region undergo a displacement generally referred to as pantographing. However, depending on the construction of the tire and on the building equipment that is used, such pantographing may not be uniform. The non-uniform displacement of the reinforcing plies may in turn affect the uniform spacing between the carcass plies in the bead area thereby resulting in what is generally referred to as ply waviness.
The tire construction of the preferred embodiment of the present invention, using axially inner and axially outer textile reinforcing layers comprising a square woven fabric, reduces the waviness of the reinforcing layers and the carcass plies and therefore improves the performance of the tire.